1. Field of the Invention
The present invention relates to a method of separating mixtures of dichloropentafluoropropanes, C.sub.3 HCl.sub.2 F.sub.5, from chloroform, CHCl.sub.3, (including but not limited to azeotropic mixtures) and recovering purified dichloropentafluoropropanes. More specifically, but not by way of limitations, the present invention relates to the selective fluorination of chloroform in the presence of dichloropentafluoropropanes to lower boiling fluorinated products (e.g., dichloromonofluoromethane) followed by separating the fluorinated products (e.g., by distillation) from the dichloropentafluoropropanes.
2. Description of Related Art
Advances in modern technology, particularly in the electronic areas, depend upon cleanliness of the various components. For example, in the manufacture of the electronic circuit boards, with their increased circuitry and component densities, thorough and effective cleaning of the boards after soldering is of utmost importance. At the present time, for the most part, cleaning of such articles is done by solvent washing utilizing various solvents and processes.
The solvent of choice at the present time is 1,1,2-trichloro-l,2,2-trifluoroethane (CFC-113) because this solvent provides the essential characteristics required of an effective washing solvent such as convenient atmospheric boiling point, non-flammability, low toxicity, inertness to various materials of construction, high stability, and high solvency. CFC-113 is often used with small amounts of co-solvents such as acetone or methanol to enhance certain solvency characteristics.
In recent years, however, CFC-113 has been suspected of contributing to the depletion of the stratospheric ozone layer. Because of its unusual high stability, it is believed that CFC-113 remains intact in the earth's atmosphere and then upon reaching the stratosphere undergoes decomposition and the decomposition products participate in the ozone layer depletion process.
In a pending U.S. patent application Ser. No. 07/422,012 filed Oct. 16, 1989, which corresponds to WIPO publication number WO91/05753, it has been proposed that each of isomeric hydrodichloropentafluoropropanes or mixtures thereof represented by the formula C.sub.3 HCl.sub.2 F.sub.5 are suitable substitutes for CFC-113. These chlorofluoropropanes include CHClFCClFCF.sub.3 (HCFC-225ba, b.p. 56.0.degree. C.), CHF.sub.2 CClFCClF.sub.2 (HCFC-225bb, b.p. 56.3.degree. C.), CHCl.sub.2 CF.sub.2 CH.sub.3 (HCFC-225ca, b.p. 53.0.degree. C.), CHClFCF.sub.2 CClF.sub.2 (HCFC-225cb, b.p. 52.0.degree. C.), and CClF.sub.2 CHClCF.sub.3 (HCFC-225da, b.p. 50.4.degree. C.). These chlorofluoropropanes have characteristics very similar to those of CFC-113, such as relatively low boiling points, non-flammability, low toxicity, inertness to various materials of construction, high solvency and in-use stability, but are believe to have little or no stratospheric ozone layer depletion potential. It is now generally believed that hydrogen-containing chlorofluorocarbons (HCFCs) undergo decomposition reactions, such as dehydrochlorination, in the atmosphere such that the compounds do not survive to reach the stratosphere and thus should have little effect upon the ozone layer depletion process.
One of the ways whereby hydrochlorofluoropropanes of the formula C.sub.3 HCl.sub.2 F.sub.5 can be prepared is by reaction of dichlorofluoromethane, CHCl.sub.2 F (HCFC-21), with tetrafluoroethylene, CF.sub.2 .dbd.CF.sub.2, in the presence of an aluminum halide catalyst as represented by the following equation: EQU CHCl.sub.2 F+CF.sub.2 .dbd.CF.sub.2 .fwdarw.C.sub.3 HCl.sub.2 F.sub.5 ( 1)
wherein "C.sub.3 HCl.sub.2 F.sub.5 " represents an isomeric mixture of hydrogen-containing chlorofluoropropanes. The reaction of CHCl.sub.2 F with CF.sub.2 .dbd.CF.sub.2 to produce these chlorofluoropropanes has been disclosed by Joyce in U.S. Pat. No. 2,462,402, by Coffman et al. in J. Amer. Chem. Soc., Vol. 71, pp 979-980 (1949), and by Paleta et al. in Coll. Czech. Chem. Comm., Vol. 35, pp 1867-1875 (1971).
One of the complications of the reaction shown in equation (1) is the concomitant production of chloroform, CHCl.sub.3, by the aluminum halide-catalyzed disproportionation of HCFC-21 or by the chlorination of HCFC-21 by the catalyst. The reaction products may contain as high as 30 mole % chloroform. In many applications, it may be desirable to have pure dichlorofluoropropane in the formulations, thus the chloroform must be removed. Theoretically, since chloroform boils at 61.2.degree. C., some 5.degree. higher than the highest boiling dichloropentafluoropropane, separations should be possible by careful distillation. However, and as made the subject of a U.S. patent application Ser. No. 07/615,912 filed concurrently herewith, chloroform forms azeotropes with most of the C.sub.3 HCl.sub.2 F.sub.5 isomers. For example, it has now been discovered that a mixture of HCFC-225 isomers consisting of HCFC-225ca, HCFC-225aa and HCFC-225cb in a 10.4:1.0:8.6 mole ratio forms a true azeotrope with chloroform. This azeotrope contains 79.8 wt. % dichloropentafluoropropane and 20.2 wt. % chloroform and boils at atmospheric pressure at 51.2.degree. C. Similarly, the individual isomers HCFC-225cb and HCFC-225ca at 71.5 wt. % and 83.6 wt. % with chloroform are established as true binary azeotropes boiling at 53.9.degree. and 50.9.degree. C., respectively. The presence of the azeotropes make the separation of the chloroform by distillation impractical.